Abstract The eukaryotic genome is packaged into chromatin formed of repeating units called nucleosomes. Nucleosomes consist of an octamer of core histone proteins (H2A, H2B, H3 and H4) around which DNA is wrapped. A fifth type of histone, the H1 linker histone, associates with chromatin in a dynamic fashion, promoting compaction of chromatin into higher order structures. Mammals express 11 H1 histone subtypes that differ in primary sequence and regulation. Post-translational modifications (PTMs) to the core histone proteins are increasingly recognized as key regulators of chromatin structure and function, affecting many processes occurring on DNA, including transcription, replication and DNA repair. Given the central role of core histone PTMs in epigenetic regulation, it is not surprising that dysregulation of their formation leads to a variety of disease states, most notably cancer. Although the H1 histone has been viewed primarily as a chromatin structural protein, studies from our lab have revealed novel roles for linker histones in epigenetic regulation, including subtype-specific regulation of core histone methylation and DNA methylation. However, the molecular mechanisms by which H1's affect core histone methylation and the full range of core histone PTMs regulated by H1 subtypes are unknown. H1 histone genes are frequently mutated in a several cancers, particularly lymphomas. However, the effects of these mutations on H1's functions in epigenetic regulation have not been studied. We propose to determine the mechanism by which certain H1 subtypes inhibit H3K4 methylation, an activating histone mark whose aberrant deposition is linked to several cancers. We also propose to explore H1-mediated regulation of methylation at other residues in H3 and H4 histones. Finally, we seek to investigate the consequences of cancer-associated mutations in H1 proteins on their functions in epigenetic regulation. Successful completion of this work will reveal novel mechanisms of epigenetic regulation by linker histones in both normal and cancer cells.